1. Field of the Invention
The present invention relates to a method and an apparatus for recording and reading out an electrostatic latent image, in which intensity distribution information of recording light is recorded as an electrostatic latent image on an electrostatic recording body.
2. Description of the Related Art
Heretofore, in medical X-ray imaging and the like, there has been known a system to be described below for reading an electrostatic latent image borne by latent image charges, that is, radiation image information. In the system, in order to decrease a radiation dosage received by a subject, to improve the diagnosis performance, and so on, a photoconductor sensitive to X-rays, for example, a selenium plate composed of amorphous selenium (a-Se), is used as an electrostatic recording body (photoconductor, solid state radiation detector). Then, recording radiation such as an X-ray bearing the radiation image information is irradiated onto this electrostatic recording body, and the latent image charges bearing the radiation image information is stored in a capacitor of the electrostatic recording body. Then, the electrostatic recording body is scanned by reading light (reading electromagnetic wave) such as a laser beam, and thus a current generated in the electrostatic recording body is detected through plane electrodes or comb electrodes on both sides of the electrostatic recording body.
In this system, the electrostatic recording body having electrodes on both of its ends and at least one photoelectric conductive layer provided therebetween is used. The recording radiation is irradiated onto the electrostatic recording body in a state where a recording voltage is applied to both of its end electrodes, and the electrostatic latent image is formed in the capacitor of the electrostatic recording body. Then, the electrodes at both ends of the electrostatic recording body are short-circuited to be set at the same potential. Further, the photoelectric conductive layer of the electrostatic recording body is scanned by means of reading light through an electrode having a transmittivity thereto (hereinafter referred to as a reading light side electrode). Thus, electric reading of the electrostatic latent image is carried out by photoinduced discharge by means of pairs (charge pairs) of electrons and holes, which are generated on an interface between the reading light side electrode and the photoelectric conductive layer. In this system, during reading of the electrostatic latent image, no current flows in the dark portions of the image, and larger currents flow in the lighter portions of the image. The system as described above, in which the electrodes at both ends of the electrostatic recording body are short-circuited after recording and larger currents flow in the lighter portions of the image, is referred to as a positive type system. The electrostatic recording body for use in this positive type system is referred to as a positive type electrostatic recording body.
As specific layer configurations of the positive type electrostatic recording body as described above, for example, there are ones to be described below. One composed of: a first conductive layer (recording light side conductive layer; the same applies to the following); a recording photoconductive layer; a trap layer as a capacitor; a reading photoconductive layer; and a second conductive layer (reading light side conductive layer; the same applies to the following) (U.S. Pat. No. 4,535,468 and the like). One composed of: a first conductive layer; a recording/reading photoconductive layer; and a second conductive layer, in which a capacitor is formed on an interface between the photoconductive layer and the second conductive layer (Medical Physics, Vol. 16, No. 1, January/February 1989; P105-109). One composed of: a first conductive layer; an insulating layer; a recording/reading photoconductive layer; and a second conductive layer, in which a capacitor is formed on an interface between the insulating layer and the photoconductive layer, and so on. Note that the first and second conductive layers are layers corresponding to the aforementioned electrodes at both ends.
Moreover, there has been proposed, as a positive type electrostatic recording body, one composed by stacking in the following order: a first conducting layer having a transmittivity to recording radiation; a recording photoconductive layer exhibiting a photoconductivity by receiving irradiation of the recording radiation; a charge transport layer functioning as an approximate insulator for charges of the same polarity as that of the charges charged on the first conductive layer and functioning as an approximate conductive layer for charges of a polarity reverse to that of the charges of the same polarity; a reading photoconductive layer exhibiting a photoconductivity by receiving irradiation of reading light (reading electromagnetic wave); and a second conductive layer having a transmittivity to the reading light, in which a capacitor is formed on an interface between the recording photoconductive layer and the charge transport layer (Japanese Unexamined Patent Publications Nos. 2000-105297, 2000-284056 and 2000-284057).
However, any of the positive type electrostatic recording bodies has a problem of so-called photovoltaic noise, in which a current flows by irradiation of the reading light even if a barrier electric field is formed on the interface between the second conductive layer having the transmittivity to the reading light and the photoconductive layer composed of a-Se or the like and a dosage of the recording radiation is in a range of 0 mR.
Moreover, in the photoconductive layer of the electrostatic recording body, though a high-resistant amorphous substance such as a-Se having traps is generally used, the following problem is inherent. From application of a voltage (generally a high voltage) between the electrodes at both ends (first and second conductive layers) of the electro static recording body to short circuit the end electrodes, charge injection occurs from the electrodes to the photoconductive layer. While the majority of injected charges are trapped as space charges, some of the charges are not trapped as space charges, and a dark current flows as a leakage current in the photoconductive layer. This dark current is stored as a dark latent image in the capacitor and emerges as a dark latent image noise in a reproduced image when being read. This dark current has a characteristic that it is large at the beginning of voltage application, decreases with time, and approaches a certain leakage current value. Specifically, a dark current level immediately after the voltage application is larger than a dark current level in a stabilized state (stabilized leakage current state). This phenomenon is more prominent as the applied voltage is higher, and in some cases, it may take, for example, 10 minutes or longer to stabilize the dark current level to the leakage current level. Furthermore, the dark current level exhibits the following tendency. Even if the dark current level is stabilized once, if the voltage application is halted for a while after the short circuit of the both electrodes, the dark current level returns to the original magnitude immediately after another voltage application performed thereafter. Hence, the dark latent image caused by the high-level dark current immediately after the voltage application becomes a large noise source when the reproduce image is read. Furthermore, a quantity of this dark latent image is varied with time from the voltage application to the irradiation of the recording radiation and with usage history. Therefore, it is also difficult to correct image data so that the dark latent image noise does not emerge on the reproduced image.
Meanwhile, there has been proposed in the above-described Japanese Unexamined Patent Publication No. 2000-105297, a method for preventing image quality deterioration, in which preexposure light is irradiated onto the reading photoconductive layer during the voltage application and before the irradiation of the recording radiation, and the dark latent image and a residual image, which are stored in the capacitor before the irradiation of the recording radiation, are decreased by utilizing the rectifying action of the charge transport layer.
Furthermore, the has also been proposed a method for decreasing photovoltaic noise, in which barriers between the charge transport layer and the recording photoconductive layer are adequately provided to somewhat form hole barriers, and the holes are stored in the hole barriers by the preexposure to make the voltage flat-band.
However, these methods are established only in the electrostatic recording body including the charge transport layer, which is described in the gazette of Japanese Unexamined Patent Publication No. 2000-105297, and the method described in the gazette of Japanese Unexamined Patent Publication No. 2000-105297 cannot be applied to the other electrostatic recording bodies described above.
Moreover, it is not easy to form hole barriers to an extent of exactly canceling the photovoltaic on the interface between the charge transport layer and the recording photoconductive layer.
Furthermore, in the case where the dark current from the reading light side electrode is originally large, and the dark latent image of the polarity reverse to that of the electrostatic latent image is formed in the capacitor, the above-described preexposure rather increases the dark latent image.
Therefore, there has been proposed in Japanese Patent Application No. 11 (1999)-194546, a method and an apparatus for recording an electrostatic latent image, which are capable of achieving the decrease of the photovoltaic noise and stabilization thereof and achieving the decrease of the dark latent image formed immediately after the voltage application and stabilization thereof in the case of using the optical reading system and the positive type electrostatic recording body. In the above-described method and apparatus, empty reading is carried out, in which the preexposure light is irradiated onto the photoconductive layer in a state where the electrode of the first conductive layer and the electrode of the second conductive layer are set at the same potential, and after the empty reading is stopped, the recording radiation is irradiated in a state where the recording voltage is applied between both electrodes to record the electrostatic latent image.